Solder printing process to reduce void formation in a microvia

ABSTRACT

In one embodiment, a method is provided. The method comprises filling a microvia formed in a bond pad with solder paste comprising solder balls of the first size; and coating the bond pad with solder paste comprising solder balls of the second size, wherein the second size is greater than the first size.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional of U.S. Ser. No. 10/976,980filed Oct. 29, 2004.

FIELD OF THE INVENTION

Embodiments of the invention relate to the fabrication of electroniccomponents, and in particular to the fabrication of high densityinterconnects, using microvia technology.

BACKGROUND

Microvia technology has enabled the development of high densityinterconnects for electronic components, and plays a crucial role inhigh density printed circuit board (PCB) and substrate design. However,one problem associated with microvia technology is the formation ofsolder voids in the solder joints formed in the microvias themselves.FIG. 1 of the drawings shows a cross sectional photograph of a solderjoint, which includes a void. Referring to FIG. 1, reference numeral 100indicates a bond pad of a printed circuit board (PCB), and referencenumeral 102 indicates a bond pad of a semiconductor die/substrate 102which is attached to the substrate. As will be seen, the bond pad 100includes a generally U-shaped microvia formed therein. A solder material104 is disposed between the bond pad 102, and the bond pad 100 andserves to electrically & mechanically connect the semiconductor die tothe substrate or package to PCB. For good electrical & mechanicalconnection, the solder material 104 is required to completely fill themicrovia in the bond pad 100. However, as can be seen from FIG. 1 of thedrawings, the microvia is not completely filled with the solder material104 as it includes an air pocket, which is solder free. The air pocketis referred to as a void, or process void. The presence of the void inthe microvia can adversely affect the mechanical and electricalproperties of a solder joined formed by the solder material 104.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional photograph of a solder joint in which asolder void may be seen;

FIG. 2 is a schematic drawing that illustrates the parameters associatedwith the filling of a microvia with solder paste;

FIG. 3 illustrates the forces acting on a volume of solder pasterequired to fill a microvia;

FIG. 4 is a table which shows how a minimum diameter D of a microviarequired to fill a microvia with solder paste, varies with the tackyforce of the solder paste;

FIG. 5 schematically illustrates the process of filling a microvia withsolder paste having particles of a first size;

FIG. 6 includes tables that show the characteristics of solder paste ofType 3, Type 4, and Type 5;

FIG. 7 schematically illustrates the filling of a microvia with solderpaste of a second size;

FIGS. 8A and 8B illustrates steps in a dual-stage solder printingprocess, in accordance with one embodiment of the invention;

FIG. 9 illustrates a solder printing operation using a printing table,in accordance with one embodiment of the invention; and

FIG. 10 shows a schematic drawing of the printing table of FIG. 9, ingreater detail.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the invention. It will be apparent, however, to oneskilled in the art that the invention can be practiced without thesespecific details. In other instances, structures and devices are shownin block diagram form in order to avoid obscuring the invention.

Reference in this specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the invention. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment, nor are separate or alternative embodimentsmutually exclusive of other embodiments. Moreover, various features aredescribed which may be exhibited by some embodiments and not by others.Similarly, various requirements are described which may be requirementsfor some embodiments but not other embodiments.

The presence of a void or “solder free pocket” within a microvia isdirectly related to the amount of solder paste that enters or fills themicrovia during solder printing. Further, the amount of solder pastethat enters or fills the microvia during solder printing is related tothe lateral dimension or diameter of the microvia. In one embodiment ofthe invention, a technique is disclosed to calculate or determine theminimum size/diameter of a microvia that is required in order toeliminate, or at least reduce void formation in a later formed solderjoint.

The various parameters used to calculate the minimum diameter are shownin FIG. 2 of the drawings. Referring to FIG. 2, reference numeral 200indicates a bond pad with a microvia 202 formed therein. As will beseen, the bond pad 200 is connected to an inner layer 204, which may,for example be an internal layer of a printed circuit board (PCB). Themicrovia 202 has a height H, and a diameter D. In FIG. 2, referencenumeral 206 indicate solder paste printed on the bond pad 200 during asolder printing operation. The solder paste 206 has a thickness T.Further, the bond pad 200 has a width diameter P.

Referring now to FIG. 3 of the drawings, the forces exerted on a volumeof solder paste 206 a required to completely fill the microvia 202 isindicated as F₁, F₂, and F₃. In order for the block 206A to completelyfill the microvia 202 the following condition has to be satisfied:F ₂ >F ₁ +F ₃

In one embodiment, the minimum diameter D for the microvia 202 in orderfor the volume 206 a to completely fill the microvia 202 may becalculated using the following equation: F₂ − F₃ − F₁ ≥ 0ρ_(s)g  V_(s) − T_(k)A_(s) − ρ_(a)g  V_(v) ≥ 0$\left. {{{\rho_{s}g\quad\left( \frac{\pi\quad D^{2}}{4} \right)T} - {T_{k}\left( {\pi\quad D\quad T}\quad \right)} - {\rho_{a}{g\left( \quad\frac{\pi\quad D^{2}}{4} \right)}H}} \geq 0}\Rightarrow{{{D\left( {\rho,{{gT} - {\rho_{a}{gH}}}} \right)} - {4T_{k}T}} \geq 0}\Rightarrow{D \geq \quad\frac{4T_{k}T}{\left( {{\rho_{s}{gT}} - {\rho_{a}{gH}}} \right)}} \right.$where

-   F₁, F₂, F₃ are force due to pressure in the microvia, weight of    solder paste above microvia and tacky force from the solder paste    respectively.-   ρ_(s), ρ_(a) are density of solder & air in microvia respectively.-   V_(s), V_(a) are volume of solder & air in microvia respectively.-   T_(k) is the tacky force of solder paste.-   T, H, D are solder paste thickness (printed), microvia height &    diameter of microvia respectively.

As will be noted, T_(k) is a key contributor to the minimum diameter Din the above equation. With H, and T set at 5 mm, the Table 400 of FIG.4 shows the minimum diameter D, at various values of T_(k). T_(k)(grams) D (millimeters) 120% D (millimeters) 40 3.7 4.4 50 4.6 5.5 605.5 6.7 70 6.5 7.8

In one embodiment, instead of using the minimum diameter D, a largervalue for example 120% of the minimum diameter D is used in order toaccommodate variances in the actual size of the microvia 202 as a resultof the fabrication process.

Using the above table, it will be seen that for a T_(k) value of around50-60 grams, 120% of the minimum diameter D is around 7 mm. Currentformulations for solder paste have a T_(k) of around 50-60 grams, andcurrent microvia designs have a diameter of 6 mm. This implies thatthere would be a certain percentage of yield loss due to the formationof solder voids in the microvias.

One advantage of predicting the minimum microvia diameter D that isrequired to completely fill a microvia with solder paste, is thatformation of solder voids in the microvia can be eliminated, or at leastreduced by altering the parameters (a) of the solder printing process or(b) the solder paste that is used in the solder printing process inorder to ensure that the actual diameter of the microvia is less than orequal to the minimum diameter D predicted by the above formula.Alternatively, the microvias themselves can be designed so that theyhave an actual diameter that is at least equal to the calculated minimumdiameter D for a given solder paste, and solder paste printing process.Accordingly, solder voids need not be detected after formation of thesolder joints, but instead can be predicted a priori, and the processparameters can be accordingly modified so as to eliminate or at leastreduce the formation of process voids in the solder joints.

Techniques for eliminating or at least reducing the formation of soldervoids in the solder joints, will now be described, with reference toFIGS. 5 to 10 of the drawings. FIG. 5 shows a bond pad 500 with amicrovia 502 formed therein. Reference numeral 504 indicates solderparticles that have been deposited in the microvia 502 during aconventional solder printing process. The solder particles 504 have acertain size in relation to the diameter of the microvia 502. FIG. 5 ofthe drawings is intended to illustrate that because of the size of thesolder particles 504 in relation to the diameter of the microvia 502, anair pocket 506 forms in the microvia 502 and prevents further solderparticles 504 entering the microvia 502. During subsequent solder jointformation, the air pocket 506 forms a solder void which weakens themechanical and electrical properties of the solder joint. In oneembodiment, a minimum of four solder particles are required to span themicrovia diameter in order to fill the microvia.

Referring now to FIG. 6 of the drawings, Table 600A indicates thevarious types of solder paste used in today's surface mount technology(SMT), in terms of their respective solder particle size. Further, Table600B indicates the number of solder particles of each solder paste typethat is required to fill a microvia having a diameter of 6 mm.

Referring to Table 600A, it will be seen that Type 3 solder pasteincludes solder particles having a size of 0.98 mm to 1.77 mm. Further,it will be seen from Table 600B, that between three and six solderparticles of Type 3 are required to fill a microvia having a diameter of6 mm.

From Tables 600A, and 600B, it will seen that the smaller particle sizesolder types would lead to a more complete filling of a microvia. Forexample, referring to FIG. 7 of the drawings, a bond pad 700 with a via702 formed therein, is shown filled with Type 4 solder paste. BecauseType 4 solder paste has a particle size that is less than the particlesize of Type 3 solder paste, more particles fill the via 702 with theresult that formation of an air pocket 706 is reduced.

However, the problem with using a smaller particle solder type is thatsolder paste with particles of a smaller size oxidize more easily thansolder paste with particles of a larger size.

In one embodiment, a dual-stage solder printing process is employed. Ina first solder printing operation, solder paste of reduced particle sizeis used to fill a microvia. For example, in one embodiment, a Type 4, ora Type 5 solder paste may be used. This is illustrated in FIG. 8A of thedrawings, where microvias 800 formed in bond pads 802 on a substrate 804are filled with a solder paste 806 comprising solder particles ofreduced size, for example solder particles of Type 4, or Type 5. It willbe seen from FIG. 8A of the drawings, that the lateral extent of theprinted solder paste 806 exceeds the size of the microvias 802. This isbecause, in accordance with one embodiment, a printing stencil withapertures larger than the diameters of the microvias 800 is used, inorder to accommodate for variances in the actual diameters of themicrovias 802 due to the manufacturing process. For example, in oneembodiment, the stencil has openings that are 2 mil larger than thedesigned via diameter. Thus, if the designed via diameter is 6 mil, thena stencil with openings of 8 mil will be used. Further, in accordancewith one embodiment, the stencil has a reduced thickness which is lessthan the conventional 5 mm to 6 mm thickness. For example, in oneembodiment, the thickness of the stencil may be equal to the thicknessof the microvia hole depth. The object of the first printing usingsolder paste of a smaller particle size is to ensure that a sufficientvolume of solder paste enters each microvia.

The next stage in the dual-stage solder printing process is to perform asecond solder printing operation, wherein solder paste having solderparticles of a larger size is used. This operation is illustrated inFIG. 8B of the drawings where it will be seen that solder paste 810having solder particles of greater size is printed over the solder paste806 which has the solder particles of reduced size. By using solderpaste having a solder particle size greater than the solder paste usedin the first solder paste printing operation, the problem of oxidationassociated with using solder paste having particles of reduced size isavoided. Thereafter, a single reflow operation may be performed in orderto ultimately form the solder joints.

In one embodiment, the solder paste used to perform the first solderprinting operation may comprise particles of Type 4, and Type 5, whereasthe solder paste used to perform the second solder paste printingoperation will comprise particles of Type 3.

In order to improve the volume of solder paste that enters a microviaduring solder paste printing, in one embodiment, the viscosity of thesolder paste used in the solder paste printing process is selectivelyreduced.

In one embodiment, the reduction in the viscosity of the solder paste isachieved by using a heating element in order to heat the solder paste asit is being printed. Accordingly, one embodiment of the inventionincludes providing a printing table that includes a heating element inorder to selectively heat an area of bond pad in the vicinity of amicrovia thereby to elevate the temperature of the solder paste beingprinted to reduce its viscosity and improve flow into the microvia. FIG.9 of the drawings illustrates this embodiment of the invention.Referring to FIG. 9, a substrate is shown supported on a printing table902. The printing table 902 is shown in further detail in FIG. 10 of thedrawings. Referring to FIG. 10, it will be seen that a printing table902 includes a support block 902A, and at least one heating element902B. The heating element 902B may be operatively connected to apower/supply. Referring to FIG. 9 of the drawings, a number of microvias904 are formed on bond pads 906. A stencil 908 having openings alignedwith the microvias 904 selectively allows solder paste to flow into themicrovias 904 when a printing component 910 is moved in the directionindicated by the arrow 912. The purpose of this heating element 902B isto selectively heat areas of the bond pad 906 adjacent to the microvias904 thereby to increase the temperature of the solder paste beingprinted and to simultaneously reduce its viscosity, to promote the flowof the solder paste into the microvias 904. Due to its thixotropiccharacteristic, the viscosity of the solder paste will eventually returnto its original viscosity. Thus, the problems associated with lowerviscosity solder paste in general, such as post printing bridgingdefects, and defects due to post printing reflow, are avoided.

In one embodiment, the heating element 902B raises the temperature ofthe solder paste in the 5° C. to 10° C. range. This temperature range ishigh enough to lead to the benefits of reduced viscosity, while at thesame time ensuring that the flux system does not evaporate at a higherrate. The flux system is important to ensure that contaminants areeffectively removed before solder joint formation.

Embodiments of the invention disclose a printing table, with an embeddedheating element, such as described above. Alternatively, the heatingelement may be a separate heating element used in conjunction with aconventional printing table, as required.

Although the present invention has been described with reference tospecific exemplary embodiments, it will be evident that the variousmodification and changes can be made to these embodiments withoutdeparting from the broader spirit of the invention as set forth in theclaims. Accordingly, the specification and drawings are to be regardedin an illustrative sense rather than in a restrictive sense.

1. A method, comprising: heating a material having a microvia formedtherein; and performing a solder paste printing operation while thematerial is heated to fill the microvia with solder paste, wherein thesolderbase comprises on of Type 4 and Type 5 solder paste.
 2. The methodof claim 1, wherein the heating is in a temperature range of between 5°C. to 10° C.
 3. A printing table for solder printing, comprising: asupport element to support a component while solder paste is printed onthe components; and a heating element mounted adjacent to the supportelement to heat the component.
 4. The printing table of claim 3, whereinthe heating element is embedded in the support element.
 5. A method,comprising: calculating a minimum diameter required for a microvia to befilled with solder paste during a solder paste printing operation,without any voids; and if the minimum diameter is greater than an actualdiameter of the microvia, then performing a modified solder pasteprinting operation to increase the volume of solder paste that fills themicrovia.
 6. The method of claim 5, wherein the modified solder pasteprinting operation comprises heating material surrounding the microviaso as to reduce the viscosity of the solder paste during printing. 7.The method of claim 5, wherein the modified solder paste printingoperation comprises a first printing operation in which solder pastehaving solder balls of a lesser size is printed; and a second printingoperation in which solder paste comprising solder balls of a greatersize is printed.
 8. The method of claim 5, further comprising performinga single reflow operation to form a solder joint in the microvia.
 9. Themethod of claim 7, wherein the solder paste used in the first solderpaste printing operation comprises one of Type 4 and Type 5 solderpaste.
 10. The method of claim 7, wherein the solder paste used in thesecond printing operation comprises Type 3 solder paste.
 11. A method,comprising: performing a modified solder paste printing operation on abond pad comprising a microvia, in order to improve a volume of solderpaste that fills the microvia wherein the modified solder paste printingoperation comprises a first printing operation in which solder pastecomprising solder balls of a lesser size is printed on the bond pad; anda second printing operation in which solder paste comprising solderballs of a greater size is printed on the bond pad and wherein thesolder paste used in the first printing operation consists of one ofType 4 and Type 5 solder paste and wherein the solder paste used for thesecond printing operation comprises type 3 solder paste.
 12. The methodof claim 11, further comprising performing a single solder reflowoperation to form a solder joint in the microvia.
 13. The method ofclaim 11, wherein the modified solder printing operation comprisesheating the bond pad to reduce the viscosity of the solder paste thatenters the microvia during the solder paste printing operation.
 14. Themethod of claim 13, wherein the heating is in a temperature range ofbetween 5° C. to 10° C.